Optical isolators have various uses in optical devices. By way of example they are used to provide unidirectional circulation of radiation in a ring laser-resonator (traveling-wave resonator) and to prevent feedback between stages of an optical amplifier. An optical-diode includes a crystal of a magneto-optic material. The magneto-optic material is used as a unidirectional polarization rotator, in conjunction with polarization-selective elements to provide the non-reciprocal transmission. The polarization rotation of the magneto-optical material is achieved by applying a magnetic field to the magneto-optic material, longitudinal in the direction of light propagation in the magneto-optic material.
It is common to have a non-reciprocal polarization rotation of 45° in one forward pass and accordingly 90° for back-reflected light. The polarization-selective elements are oriented in 45° with respect to each other, resulting in optical isolation of the back-reflected light. A deviation of the rotation angle has direct impact on the optical isolation performance
Optical-isolators are most effective in a wavelength range between about 400 nanometers (nm) and 1100 nm. The effectiveness of an optical-isolator depends on a so-called “Verdet” constant of the magneto-optic material. This constant defines a degree of polarization-rotation, per unit length of the material, per unit applied magnetic field. The most widely used magneto-optic material for optical isolators is terbium gallium garnet (TGG) which has a relatively high Verdet constant compared with that of other magneto-optic materials. Polarization rotation provided by TGG is particularly temperature sensitive. Because of this, an optical isolator including TGG usually requires some form of temperature control to optimize optical isolation even under high power irradiation and under environmental changes.
A TGG crystal for use in an optical isolator is relatively expensive and contributes significantly to the cost of an optical isolator. Further, TGG, while nominally transparent to radiation in the above-referenced wavelength range, has a finite absorption for that radiation. The absorption can result in significant heating of the crystal in a case where high-power radiation is being transmitted by the crystal.
The higher the magnetic field that can be applied to a TGG crystal the smaller (shorter) the crystal needs to be to provide a required polarization rotation. The smaller the crystal, the less expensive the crystal will be, and the less the absorption of radiation will be.
One particularly effective arrangement for providing a high magnetic field in a crystal of a magneto-optic material is described in U.S. Pat. No. 7,206,166. Here, the magnetic field is provided by an effectively cylindrical arrangement of permanent magnets. The effectively cylindrical arrangement includes a central magnet which is an actual cylinder which is axially magnetized. The magnetic field of the cylinder extends within the cylinder, approximately parallel to the axis of symmetry of the cylinder, in only one direction from the north-pole to the south-pole. A roller-shaped magneto-optic crystal is arranged within the cylinder.
Terminal magnets are attached to each of the two end faces of the central magnet in a plane perpendicular to the axis of symmetry. Each of the terminal magnets is configured as a hollow cylinder and has a through-aperture in the extension of the axis of symmetry. Each terminal magnet is largely radially magnetized with regard to the axis of symmetry. One of the two terminal magnets is magnetized radially from interior to exterior and the other terminal magnet is magnetized radially from exterior to interior. Each of the terminal magnets is formed from a plurality of wedge-shaped magnets for effecting the radial magnetization of the terminal magnets.
While the arrangement of the '116 patent may be highly effective in providing a concentrated magnetic field, the arrangement has significant shortcomings. The cylindrical center magnet and the wedge-shaped magnets forming the terminal magnets will be expensive to produce compared with simple bar-magnets. The cylindrical assembly of magnets restricts direct thermal access to the magneto-optic crystal. Accordingly, thermal control of the magneto-optic crystal must be provided by placing the entire magnet assembly, with the magneto-optic crystal therein, inside a thermally controlled enclosure.
Such an enclosure would be relatively expensive and would require a relatively large power supply. Cost aside, however, control by such a large enclosure would have a very slow response time due to the large thermal mass of the magnet assembly, which could be over one-hundred times greater than the thermal mass of the magneto-optic crystal. There is a need for a magnet assembly capable of providing a magnetic field comparable to that of the '116 patent but which provides direct thermal access to the magneto-optic crystal, allowing the crystal temperature to be controlled independent of the magnets and with relatively fast response. Preferably the magnet assembly should be formed from simple bar-magnets for economy of construction.